• Chinese Journal of Lasers
  • Vol. 48, Issue 20, 2010002 (2021)
Kaipeng Li1、2, Yan He1、*, Chunhe Hou1, Jian Ma1, Zhengyang Jiang1, Weibiao Chen1、**, Peng Chen3, Fanghua Liu1、2, Yongqiang Chen1、2, and Shouchuan Guo1、2
Author Affiliations
  • 1Key Laboratory of Space Laser Communication and Detection Technology, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China
  • 2Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China
  • 3State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou, Zhejiang 310012, China
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    DOI: 10.3788/CJL202148.2010002 Cite this Article Set citation alerts
    Kaipeng Li, Yan He, Chunhe Hou, Jian Ma, Zhengyang Jiang, Weibiao Chen, Peng Chen, Fanghua Liu, Yongqiang Chen, Shouchuan Guo. Detection of Chlorophyll Profiles from Coastal to Oceanic Water by Dual-Wavelength Ocean Lidar[J]. Chinese Journal of Lasers, 2021, 48(20): 2010002 Copy Citation Text show less

    Abstract

    Objective Marine phytoplankton is crucial to the marine ecosystem due to their important roles in the global primary carbon cycle. They can also be used as an evaluation criterion of water property because of their correction to the watercolor remote sensing parameters. Two detection schemes are often used to obtain the distribution of subsurface phytoplankton, including passive remote sensing technology, such as ocean color remote sensing technology, and activity detection technology, for instance, ocean lidar. The ocean color remote sensing is a practical approach to detecting chlorophyll-a (Chl-a) which is a proxy of phytoplankton. However, as a passive remote sensing scheme, the ocean color remote sensing system can only obtain the integral information of upper waters. The vertical distribution of subsurface phytoplankton in the ocean is significantly important for ocean remote sensing study. To obtain the depth-resolved profiles of Chl-a concentration, it is necessary to use the active detection system, such as the ocean lidar system. The conventional ocean lidar systems are often equipped with 532 nm lasers; the 532 nm laser is robust and cost effective and can penetrate ocean water with high-energy pulses. However, a blue laser penetrates and detects clean ocean waters better than a green laser with the same pulse energy. This study presents a novel dual-wavelength ocean lidar (DWOL) system equipped with a dual-wavelength laser, which can emit 532 and 486 nm lasers simultaneously. An airborne experiment and a shipborne in-situ experiment were conducted in the South China Sea to validate the performance of DWOL; the airborne data has been processed to inverse the vertical Chl-a concentration. We hope that our novel DWOL system can be useful in researching the vertical distribution of the subsurface phytoplankton concentration in the South China Sea.

    Methods The flying speed was around 55 m/s; given that the repetition rate of the laser is 100 Hz, 500 frames of airborne data obtained in 5 s will be accumulated to extend the dynamic range of the detector system. This study will use the Klett method to inverse the airborne data from offshore water to coastal water. The attenuation coefficient of 486 nm and 532 nm channels was retrieved with the Klett method to compare the difference in coefficients between coastal water and oceanic water. The detectors equipped in the DWOL are photomultiplier tube (PMT) detectors. The PMTs will generate a series of after-pulse count (APC) noises after receiving strong returning signals from the upper water, which will mislead the inversion results from the deep water. To avoid the effect of APC and the saturated data returning from the water surface, the airborne data from 80 m to 6 m underwater will be used to analyze the distribution of subsurface chlorophyll concentration. The correction between the shipborne inversed results and the in-situ measurement results will be analyzed to validate the effectiveness of the inversed airborne results. Finally, the swath of about 120 km was selected, and the Chl-a concentration based on the airborne data from the selected swath was inversed to analyze the distribution of Chl-a in the South China Sea.

    Results and Discussions The profiles of backscattering signals recorded with 486 nm channel and 532 nm channel in continuous 5 s were accumulated to enhance the dynamic range of the detecting system after the effect of after-pulse noise induced by the strong signals from upper water was removed (Fig. 4). The results [Fig. 4(a)] show that the backscattering signals obtained with 486 nm channel are about 25% deeper than those obtained with 532 nm channel, although the attenuation trend in coastal water is almost the same [Fig. 4(b)]. The inversed results of airborne data in coastal water and oceanic water (Fig. 5) show that the attenuation coefficient of 532 nm channel is larger than that of 486 nm channel, which means the 486 nm laser is more suitable for detecting oceanic water. However, the attenuation coefficients of 486 nm and 532 nm channels are virtually equal in coastal water, indicating that the 532 nm laser becomes more suitable for coastal water detection. The asynchronous shipborne experiment was conducted in coastal water and oceanic water. The synchronous shipborne measurement results show that, in oceanic water, the maximum chlorophyll concentration is at 60 m underwater (Fig. 7); the maximum chlorophyll concentration in coastal water ranges from 40 to 20 m underwater (Fig. 6). The correction analysis between the inversed airborne results and the in-situ shipborne results shows that the determination coefficient of correlation between the inversed airborne data and the in-situ shipborne data is above 0.8 (Fig. 9), indicating that the inversed airborne data and the in-situ shipborne measurement results are in good agreement. A flight swath was selected from the tracks, which began the offshore water Swath-A to the coastal water Swath-B and was about 120 km in total. The inversed results of chlorophyll concentration based on the airborne from the selected swath are shown in Fig. 10. The result (Fig.10) shows that the subsurface chlorophyll scattering layer is meanly suspended at 60 m underwater in the offshore water. When the tracks are near coastal water, the subsurface chlorophyll scattering layer raises rapidly and is suspended from 40 to 20 m underwater because the water becomes shallower near the coastal line.

    Conclusions In this study, we describe a unique dual-wavelength lidar system that simultaneously uses 532 and 486 nm lasers. An aerial experiment and a synchronized shipborne in-situ measurement experiment were undertaken in the South China Sea to test the performance of the dual-wavelength lidar. The inversed findings of attenuation coefficients for the 486 nm and 532 nm channels reveal that the 486 nm blue laser is better for detecting oceanic water, whereas the 532 nm green laser is better for detecting coastal water. The continuous inversed results of Chl-a concentration from coastal water to oceanic water show that the subsurface chlorophyll scattering layer in the oceanic water is primarily suspended at 60 m underwater. In contrast, the chlorophyll scattering layer in coastal water is mainly suspended between 40 m to 20 m underwater. The determination coefficient of correlation between the inversed aerial results and the in-situ shipborne measurement results is more significant than 0.8, showing compelling inversed airborne results.

    Kaipeng Li, Yan He, Chunhe Hou, Jian Ma, Zhengyang Jiang, Weibiao Chen, Peng Chen, Fanghua Liu, Yongqiang Chen, Shouchuan Guo. Detection of Chlorophyll Profiles from Coastal to Oceanic Water by Dual-Wavelength Ocean Lidar[J]. Chinese Journal of Lasers, 2021, 48(20): 2010002
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